Rotary filters of the above-mentioned type are commonly used for separating fibers from a fiber suspension. The filter comprises a group of annular filter discs each having a series of filter elements, the discs being mounted in substantially parallel, axially spaced relation on a horizontal drive shaft in a container for the fiber suspension. Each filter element is a hollow body having walls of screen cloth and communicating at its radially inner end with a discharge tube for filtrate. As the shaft and its discs rotate, the hollow filter elements pass successively through the suspension in the container, where filtrate enters the filter elements while more or less dewatered fiber cakes adhere to the outside of these elements. At least one nozzle for spraying a high pressure fluid is located at each side of each filter disc in a removal zone through which the filter elements pass after emerging from the fiber suspension. The sprays from these nozzles are intended to remove the filter cakes from the filter elements.
Rotary filters of the above-described type will normally have 5 to 20 filter discs each divided into 12 to 20 filter elements in the form of sectors which are truncated at their radially inner ends. In the operation of such filters when dewatering fiber pulp suspensions, a high capacity is desired, which means a high rotational speed. Also, it is desired to provide the dewatered fiber cake with a high dry solids content, which is achieved if the fiber cake is allowed to remain on the outside of each filter disc as long as possible from the moment when the disc carries the fiber cake from the suspension as the disc rotates.
If spray nozzles are provided in the conventional manner for spraying off the fiber cakes on both sides of each filter disc, some applications at a high rotational speed can give rise to problems, as the fiber cakes may adhere so firmly to the filter elements that parts of same are not removed but follow the filter disc in its rotation, so that the capacity of the filter is reduced. The reason is that the fiber cake in such cases is folded outwards at its upper part and is gradually rolled off the filter element, which takes a relatively long time.